Taking wave soldering machines as an example, these serve traditionally in the soldering of electronic components on circuits (whether components inserted in the circuit or components surface mounted on the circuit), as well as for tinning electronic component connections and even for brazing contact strips onto electronic supports such as hybrid circuits.
The design of these machines is such that components to be brazed or tin-coated are brought into contact with one or several waves of molten solder obtained by means of pumping through a jet, the solder bath contained in a vat.
Initially the circuits (on which components have been reported) or components to be tin-coated will generally have been fluxed in an upstream area of the machine, mainly so as to deoxidize the metal surfaces to facilitate their eventual wetting by the solder, the fluxing operation being followed by a so-called pre-heating operation, carried out to activate the fluxes previously deposited on the circuit or component, and to preheat the circuits or components before they arrive in the soldering hot zone. Application of such a chemical flux usually gives rise to the need after brazing or tin-plating for the user to proceed with a product cleaning operation, frequently with the aid of chlorinated solvents, ensuring the elimination of flux residues still remaining on the circuit or component. The machines are traditionally open to ambient air atmosphere.
Among the problems encountered by the users of such machines, the following may be quoted:
the formation of oxide layers (known as dross) on the surface of the solder bath as a result of its exposure to ambient air, leading to a not inconsiderable loss of solder and the need for regular cleaning of the bath. A medium sized machine is likely to produce up to 1 kg of dross per hour of service.
the second problem is connected with the cleaning operation after soldering. Within the framework of the "Montreal protocol", these solvents are subject to extremely strict regulations, and in some instances even subject to a pure and simple ban according to countries.
As a solution to these dross-formation and cleaning problems, a nitrogen cover has been proposed, at least above the solder bath, though in some cases throughout the rest of the machine, this being combined with the use of low activity flux leaving a minimal level of residue on the circuits, thus allowing the subsequent cleaning operation to be completely dispensed with, the nitrogen cover above the bath also greatly reducing the oxidation process of the solder bath.
It was thus deemed important to achieve a gas injection allowing the best compromise of performance and cost-effectiveness, between the residual level of oxygen achieved at least above the solder bath, of the gas consumption and of the machine production rate involving board input and output at a specific rate, the board input/output movements obviously being a source of pollution of the atmosphere due to air intake.
Machines made fully inert have thus appeared, designed from the start as a sealed tunnel, but also hood systems allowing a nitrogen cover to be provided for existing (open-bath) machines (retrofitting), at least for the solder bath, but also at times over part of the pre-heat zone and in the cooling zone downstream from the bath.
Performances claimed for the two types of system show that the more the cover is restricted in length (for instance exclusively limited to the solder bath), the more difficult it is to maintain in the hood a reasonable amount of nitrogen flow, while achieving very low residual oxygen concentrations (for instance 10 ppm of residual oxygen or less). A "short" system compared with the machine length, will effectively accommodate with greater difficulty the air intakes connected with board input/output, hence the need to maintain a high nitrogen flow.
In certain machines, atmospheres made inert by flow rates of the order of 50 or 60 m.sup.3 /hour have appeared, often incompatible with the user's economic constraints (and the balance sheet).
Such mediocre performances are very clearly associated with imperfect and non-optimised control over the manner whereby gas is injected into the hoods.
In that context, the applicant proposes in U.S. Pat. No. 5,161,727, an inert-rendering arrangement comprising a set of hoods defining at least at the level of the solder bath, an internal space separated from the surrounding atmosphere by means of seals, gas injection ducts opening onto the upper part of hoods and fitted with diffuser means.